And, no I'm not creating records using webhooks. Just manually in the user interface.

Yes, indexed is a field that must be unique within the table and it speeds up retrieval substantiallyl.See https://www.linkedin.com/pulse/what-indexing-types-rdbms-nishchal-vaishnav-yewxf/

Is there any way to prevent the new record from being committed in cases where there is already a value for the index field?